Low-Cost Magnesium-Based Thermoelectric Materials: Progress, Challenges, and Enhancements

Magnesium-based thermoelectric (TE) materials have attracted considerable interest due to their high ZT values, coupled with their low cost, widespread availability, nontoxicity, and low density. In this review, we provide a succinct overview of the advances and strategies pertaining to the development of Mg-based materials aimed at enhancing their performance. Following this, we delve into the major challenges posed by the severe working conditions, such as high temperature and thermal cycling, which adversely impact the behavior and long-term stability of the TE modules. Challenges include issues like the lack of mechanical strength, chemical instability, and unreliable contact. Subsequently, we focus on the key methodologies aimed at addressing these challenges to facilitate the broader application of the TE modules. These include boosting the mechanical strength, especially the toughness, through grain refining and additions of second phases. Furthermore, strategies targeted at enhancing the chemical stability through coatings and modifying the microstructure, as well as improving the contact design and materials, are discussed. In the end, we highlight the perspectives for boosting the practical applications of Mg-based TE materials in the future.


INTRODUCTION
The depletion of natural resources and environmental degradation necessitate the adoption of clean and renewable energies to reduce dependence on fossil fuels.Simultaneously, a significant portion of the world's energy is squandered in the form of heat emissions, including vehicle exhaust, industry steam boiler exhaust, overheated solar panels, and home appliances like air-conditioning/refrigeration condensers, ovens, and computers. 1hermoelectric (TE) technology converts heat directly into electricity via the Seebeck effect, created by the temperature difference between hot and cold ends.This process requires nonmoving parts, emits zero-emission, and ensures long-steady operation, making it promising for waste heat retrieval, power generation, and refrigeration. 2 Moreover, TE technology can enhance the efficiency of utilizing traditional fossil fuels by converting waste heat back into electricity, contributing to energy savings and environmental preservation.The merit of a TE material can be assessed by the dimensionless figure of merit, ZT = S 2 σT/κ, where S stands for the Seebeck coefficient, σ is the electrical conductivity, T represents the temperature, and κ is the thermal conductivity. 2heoretical calculations suggest the maximum ZT value can reach 14 for Bi 2 Te 3 -based nanowires materials. 3Additionally, thin-film Heusler alloys based on Fe 2 V 0.8 W 0.2 Al, fabricated by magnetron sputtering, can theoretically achieve a maximum ZT of 7 between 300 and 400 K. 4 In practical form, the ZT values of the materials have been significantly improved toward 3 via techniques such as band engineering, doping, and artificial micro−nanostructure processing by modulating the thermal conductivities, electrical conductivities, and Seebeck coefficients of the materials.Experimental measurements have shown that the ZT value of N-type SnSe hit 2.8 at 773 K. 5 Layered flake Cu 1.94 Al 0.02 Se, prepared by DC hot pressing process, obtained a ZT value of 2.62 at 1029 K. 6 Kim et al. claimed a high ZT of 1.86 at 320 K for Bi 0.5 Sb 1.5 Te 3 synthesized by liquid-phase compression. 7Significant advances have been made in TE materials with high performance across a wide temperature range since the early 2000s. 8,9−14 However, these compounds often contain toxic elements like Pb and expensive, rare elements such as Te, In, Hf, and Bi.Additionally, common PbTe, CoSb 3 , and Bi 2 Te 3 have high mass densities between 6.5 and 8.5 g•cm −3 , potentially hindering their application, particularly in industries like automotive and aerospace. 15Attributes such as low density, plastic deformation, and high fracture toughness 16 are critical for TE applications to withstand mechanical stress from vibrations and thermal cycling. 17agnesium-based TE materials offer a promising alternative, utilizing less expensive, nontoxic, and earth-abundant materials to meet environmental regulations and gain widespread acceptance in the energy market. 18While the current cost of TE system (above 10 $/W) is higher than other clean powergeneration technologies like photovoltaics ($0.5/W) and wind power ($0.4/W), 19,20 the cost of raw materials significantly impacts TE modules costs.Mg 2 Si 0.6 Sn 0.4 , for instance, has a material cost of about $4/kg, much cheaper than commercialized TE materials such as Bi 2 Te 3 , PbTe, and SiGe, with expenses of $110/kg, $81/kg, and $371/kg, respectively. 17oreover, Mg-based materials have low density (ρ = 1.98− 2.76 g•cm −3 ), providing an advantage in the specific figure of merit (ZT/ρ) over other commercial TE materials like Skutterudites and PbTe, crucial for applications where weight is a consideration, such as airborne and motion devices. 21olymeris et al. reviewed the micro-and nanostructure properties, as well as the role of alloying in the development of Mg 2 Si based TE materials. 22Zhou et al. summarized the recent advances of Mg-based thermoelectric, including Mg 2 X (X = Si, Ge, Sn), Mg 3 (Sb,Bi) 2 , and α-MgAgSb, from both material and device level. 23They also analyzed the strategies to maximize their ZT values and the conversion efficiency from modifying their electronic band structures, crystal structures, and thermal and electrical transport properties.Similar principles for Mg 3 Sb 2 and its derivatives was outlined by Shi et al. 24 Han et al. summarized the progress of magnesium-based energy materials in 2023. 25However, there has been less emphasis on studying the mechanical and chemical stability of these materials. 22In this review, we first highlight recent advances in Mg-based TE materials (Section 2).Subsequently, we identify the major challenges and hurdles encountered in the application of TE techniques (Section 3), such as brittleness (Section 3.1), susceptibility to oxidation (Section 3.2), and high contact resistance (Section 3.3).We then outline proposed solutions to address these issues, which include toughing the TE materials through grain refinement and addition of second phases (Section 4.1), protecting the TE legs with various coatings (Section 4.2), improving contact by modifying the design and carefully choosing contact materials (Section 4.3), etc.After briefly introducing the progress of the magnesium-based thermoelectric generator (TEG) modules (Section 5), we finally conclude with some perspectives on the future development of Mg-based TE materials and modules (Section 6).

DEVELOPMENT OF THE LOW-COST MG-BASED THERMOELECTRIC MATERIALS
The binary compounds of Mg 2 X (X: Si, Ge, Sn) exhibit a facecentered cubic (FCC) crystal structure, with the unit cell composed of 12 atoms. 26In this structure, the X 4− ions occupy the four facecentered cubic positions, while the Mg 2+ ions occupy the eight centered tetrahedral sites (Figure 1a).Mg 2 Si possesses a narrow bandgap and exhibits N-type conductivity, primarily due to native defects.It demonstrated both a high electronic conductivity and a high Seebeck coefficient, with a peak figure of merit, ZT, ranging from 0.6 to 0.8. 27Additionally, its high melting point (∼1358 K) and excellent thermophysical properties, including boosted compression strength (1640 MPa) and Young's modulus (120 GPa), along with a low thermal expansion coefficient, make it fit for a wide range of environmental applications. 22Alternatively, Mg 2 Sn and Mg 2 Ge have lower peak ZT value, which may not be sufficient for practical TE module applications. 28For instance, the maximum ZT for N-type Mg 2 Ge, fabricated through reaction of MgH 2 and Ge via spark plasma sintering (SPS), reaches only 0.32. 29he intermetallic compounds such as Mg 2 Si, Mg 2 Ge, and Mg 2 Sn demonstrate higher thermal conductivity than those traditional BiTebased and PbTe-based TE materials, which impose limitations on their maximum ZT value.The coupling between thermal and electrical transport properties makes it challenging to enhance the TE performance of Mg 2 X binary compounds, as these properties are directly influenced by carrier concentration.Furthermore, the concentration of majority carriers has essential influence on the electrical transport properties, especially at high temperatures where the bipolar effect may come to play.Mg 2 Si-based materials offer carrier controllability through impurity doping, providing design flexibility in adjusting thermal impedance.Thus, optimizing carrier concentration for maximal TE performance is vital, achievable through band structure engineering techniques such as doping the matrix with acceptor or donor elements to achieve band convergence, forming solid solutions, introducing point defects, and modifying the nanostructure to influence electrical conductivity and Seebeck coefficient. 23,33xtensive interest and attention have been directed toward the development of the Mg 2 X-based TE materials. 34,35Alloying Sn and Ge in Mg 2 Si can introduce point defects that induce short-wavelength photon scattering 23 and mass difference scattering due to the large mass difference between the elements Si and Sn, thereby reducing thermal conductivity.For example, alloying with Sn has resulted in reported peak ZT-values of 1.4 for Mg 2 Si−Mg 2 Sn solid solutions, attributed to expanded valley degeneracy and decreased lattice thermal conductivity in the alloys. 36Additionally, adjusting the composition to achieve band convergence can lead to higher carrier mobility and power factor (S 2 σ).Complete band convergence has been reported in the range of x = 0.6−0.7 for Mg 2 Si 1−x Sn x . 18imilarly, alloying with Ge in N-type Mg 2 Si 1−x Ge x solid solutions can improve ZT value to above 1.0, although slightly less effective than Sn addition. 23Various combinations of alloying elements (Si/Sn/Ge) with magnesium have been explored to enhance their TE properties as summarized in Figure 2a/b. 17Furthermore, other elements such as Cu, Ag, Ni, Zn, and In have been considered as acceptor dopants in Mg 2 Sn, albeit with limited success. 33rther doping of Bi 37 and Sb 38 into the Mg 2 X alloy system increased the maximum ZT values to 1.4 and 1.5, respectively, at 800 K. 36 These advancements have spurred the development of another group of alloys based on Mg 3 Sb 2 /Mg 3 Bi 2 Zintl compounds and MgAgSb.Mg 3 Sb 2 compounds possess a typical cubic antibixbyite symmetry of the minerals, with dynamically stable Ia3̅ and P3̅ m1 phases at ambient conditions. 31The P3̅ m1 phase of Mg 3 Sb 2 (Figure 1b) attracts much attention for its good TE performance.Mg 3 Bi 2 shares the same crystal structure as Mg 3 Sb 2 , and forms a complete solid solution within the entire composition range. 39N-type Mg 3 Sb 2 has demonstrated excellent TE performance due to much higher carrier mobility and larger band degeneracy, achieving a peak ZT of 1.5 in N-type Mg 3 Sb 2 -based materials at around 700 K through chemical doping, slightly excess Mg addition, and microstructure engineering. 40,41By alloying with Bi, the band gap of Mg 3 Sb 2-x Bi x is reduced, leading to excellent TE performance at near-roomtemperature, enabling its application as a solid-state cooling alternative. 42Doping of Mn (Mg 3.2−x Mn x Sb 1.5 Bi 0.49 Se 0.01 , x = 0.01) enhances the carrier concentration and mobility from 3.92 × 10 19 cm −3 and 9.85 cm 2 V −1 s −1 (Mn free), to 4.23 × 10 19 cm −3 and 29.50 cm 2 V −1 s −1 , respectively. 43Meanwhile, a peak ZT value of 1.6 was obtained at 723 K with the Mn doping (x = 0.02).By incorporation metallic inclusions such as Nb or Ta into the Mg 3 (Sb,Bi) 2 -based matrix, the electrical conductivity was enhanced and the lattice thermal conductivity was reduced, leading to a record-high average ZT > 1.5 with a maximum value of 2.04 at 798 K. 44 Other alloying elements like Y, Sc, Se, Te, Tm, and Nd have also been found to boost the ZT value to 1.8−1.9,especially at elevated temperatures, but are restricted by their scarcity. 23,45,46However, the single valence valley with low carrier mobility at the Brillouin center results in a low power factor of the hole-doped Mg 3 Sb 2 , making it a poor P-type TE material despite its low lattice thermal conductivity. 47ortunately, the P-type α-MgAgSb exhibits phonon glass electron crystal behavior, and a peak ZT value of 1.2 has been obtained by enhancing the phase purity of the hole-doped sample. 48The complex lattice structure of α-MgAgSb consists of a distorted Mg−Sb rock salt lattice rotated by 45°along the c axis and silver atoms inside the polyhedrons as shown in Figure 1c. 32,49Due to the natural point defects and the disordered rock salt sublattice, α-MgAgSb inherently possesses low lattice thermal conductivity, ranging from 0.51 to 0.76 W•m −1 •K −1 at room temperature.Native Ag vacancies are the main intrinsic point defects in α-MgAgSb, leading to significant strain fluctuations and mass differences in lattice, which enhance phonon scattering.A high peak ZT of 1.3 and an average ZT of 1.1 in the temperature range of 300−500 K have been achieved in α-MgAgSb. 50hemical doping-induced point defects by Yb and other elements like Ni or Zn in α-MgAgSb can also produce more phonon scattering, further suppressing the lattice thermal conductivity and improving ZT to 1.4 as shown in Table 1. 51α-MgAgSb stables at temperatures <560 K, limiting its application to a narrow temperature range for special structures. 52,53   and tetradymites demonstrating its competitiveness in the midtemperature range. 23part from the bulk materials development, the performance of the Mg-based coatings was also investigated.Mg 2 Si thin films were prepared by thermal evaporation of Mg and subsequent annealing at 623 K, which suppressed evaporation of Mg, decomposition of Mg 2 Si and oxidation of Mg 2 Si.A relatively high Seebeck coefficient of −235 μV•K −1 and a low thermal conductivity of 1.4−1.7 W•m −1 •K −1 were obtained at 712 K for the polycrystalline Mg 2 Si thin film, resulting in a ZT of 0.68. 54Stochiometric Mg 2 Sn coatings, deposited by magnetron cosputtering of Mg an Sn with separated targets, achieved the best figure of merit, ZT = 0.27, at 473 K. 33 Although the ZT value is generally lower than that of the bulk material, the lesser amount of material used provides them with an economic advantage.

CHALLENGES AND MAJOR ISSUES OF THE MAGNESIUM-BASED TE MATERIALS AND MODULES
Pursuing high ZT through the implementation of diverse phonon engineering and electron engineering schemes has been a focal point of the entire TE community.However, large electric fields, high thermal gradient, and elevated working temperature require high thermal stability to withstand temperature fluctuations and maintain materials composition, microstructure, and properties.There are a few barriers to bring these materials and technologies to market, such as inadequate mechanical properties, insufficient thermal stability, unreliable contacts, and the lack of matched P-type based materials. 66.1.Mechanical Failure.Considering the operating conditions encountered during TE generation, stress is primarily induced by the large temperature difference between the hot and cold sides of the device.Additionally, the mismatch of coefficient of thermal expansion (CTE) between TE legs and contact materials would lead to extra stress.These create unique conditions for the deformation of TE legs and other elements of TE modules, which differ from those encountered under uniform heating.Furthermore, shock loads, vibration, and cyclic temperature effects in mobile applications can adversely affect the device's integrity and stability. 12Thermomechanical properties are vital for the production and consistent functionality of TEG materials.
Inorganic TE semiconductor materials are brittle due to the crystalline structure, which contains intrinsic ionic, covalent, and/or van der Waals bonds, allowing easy cleavage along the ab-plane. 67As shown in Figure 3a, we found that the Mg 2 Si 0.4 Sn 0.6 pellet were very easy to break after sintering if not controlled properly due to its low mechanical strength.TE materials are typically polycrystalline samples produced through melting or powder metallurgy methods, which inevitably contain a high concentration of defects or flaws, leading to reduced mechanical strength. 68The flaws limiting strength include volume type (e.g., pores/cavities, agglomerates, porous regions, inclusions, and large grains), surface type (e.g., handling damage, machining damage, pitting, oxidation, and chemical product), and edge type (e.g., edge chipping). 69As shown in Figure 3b, the Mgdeficient compound Mg 1.9 (SiSn) sintered at 973 K for 4 h in a vacuum after mechanical alloying had different defects like elemental agglomeration (Si), pores, etc. 70 These defects lead to unpredictable changes or degradation in TE performance.Meanwhile, suitable mechanical strength and hardness are crucial to prevent surface damage during handling, and leg production such as the cutting and packing process.TE legs also require sufficient toughness to enhance productivity and prevent failure due to thermal fatigue or thermal shock during the repetitive heating and cooling cycles encountered in practical applications.The conventional TE device consists of bulk TE materials and electrodes through a rigid connection.During service, the accumulation of structural defects, external shear stress, and thermal stress can cause cracking, warpage, and the mechanical destruction of TE legs, contact structures, and other elements of modules, leading to a surge in internal resistance.Due to the limited compressive strength, the TE material may also fracture, leading to device failure. 69he elastic modulus of the Mg 2 Si-based TE materials ranges from 58 to 145 GPa, with their corresponding hardness values ranges from 2.4 to 5.6 GPa, which are higher than those of Bi 2 Te 3 (32−52 GPa) and PbTe (27−58 GPa) based TE materials. 71,72These values are affected by differences in grain size according to the production methods, i.e., induction melted casting or spark plasma sintering.The Mg 2 Sn compound shows a Young's modulus of 82 GPa and a hardness of 1.7 GPa. 72The formation of defects, such as pores and cracks can reduce the elastic modulus. 73The modulus reduces with increasing temperature and easily deforms above the yield stress at elevated temperatures.Mg 2 Si 1−x Sn x solid solutions exhibit a reasonable value of Vickers hardness (3.07−3.54GPa) but demonstrate low fracture toughness in a range of 0.64−1.0MPa• m 1/2 . 16,72,74This value falls between the lowest (0.35 MP•m 1/2 ) for PbTe and the highest (2.8 MPa•m 1/2 ) for Ca 3 Co 4 O 9 , 71 and is comparable to the soda-lime glass (0.7−0.8 MP•m 1/2 ). 75The bending strength of Sb-doped Mg 2 Si fabricated by SPS is 57 MPa at room temperature, 73 and the tensile strength for Mg 2 Si with glass inclusions was ∼2 MPa. 76hen TE modules are subjected to mechanical and thermal stresses, defects experience significantly higher stress compared to the average stress.This is because cracks propagate easily in brittle materials, even under low external loads.No noticeable plastic deformation occurs before fracture, and cracks develop due to thermomechanical stress when operating at high temperatures.Mg 2 Sibased TE materials are brittle with poor fracture toughness and flexure strength, so improving their toughness is a critical issue for practical usage in power generation. 77,78.2.Chemical Stability of the Materials.−83

Oxidation.
Magnesium and its alloys undergo catastrophic oxidation at temperatures above 673 K, and silicon begins to oxidize rapidly at a temperature of 973 K. 84 The Mg 2 Si surface is highly reactive due to the existence of the Mg, resulting in a low standard Gibbs free energy of formation of MgO. 73Inoue et al. reported that Mg(OH) 2 and MgH 2 were formed on the Mg 2 Si powder surface in air at room temperature, followed by the formation of Mg(CO) 3 layer with CO 2 on Mg(OH) 2 layer.Upon heating to 573 and 623 K, these compounds decompose, and Mg 2 Si starts to oxidize above 753 K (eq 1), with the Mg 2 SiO 3 phase forming at 873 K (eq 2). 85 (1) As shown in the TGA test of Mg 2 Si in the work of Park et al. (Figure 4a), the weight slightly reduced from around 573 K due to the removal of residual moisture or the sublimation of elemental Mg, which has a relatively high vapor pressure. 86A significant weight gain appeared at about 773 K and increased sharply up to 973 K, leading to the gradual oxidation of the surfaces of the Mg 2 Si.The dark gray color of the Mg 2 Si pellet surface changed to dark yellow (inset picture in Figure 4a) after 1h exposure to air at 973 K.
Tani et al. reported that the Mg 2 Si reacted with O 2 in the air above 723 K to yield MgO and Si, forming an 8 μm thick oxide layer on the surface after heat treatment at 873 K for 3 h. 87The oxidation is diffusion-controlled with an activation energy of 177 kJ/mol, calculated using test data between 773 to 923 K.Only MgO was formed on the evaporatively deposited Mg 2 Si film at lower temperatures, while SiO 2 started to form when the temperature rose above 983 K, and MgSi 2 O 4 was found in the oxide scale when the temperature reached 1213 K. 27 The MgO layer on Mg 2 Si-based TE materials was a few nm thick under ambient conditions and increased to approximately 1 μm at elevated temperatures, providing protection up to temperatures around 723 K in air. 27,88The oxidation followed parabolic kinetics, and the oxide growth was controlled by outward Mg 2+ diffusion through MgO. 27The onset (ignition) temperature of catastrophic oxidation for Mg 2 Si was about 1313 K. 27 The oxidation of tin speeds up at 423 K, and Mg 2 Sn forms no passivating layer at high temperatures.The ignition temperature of the catastrophic oxidation for Mg 2 Sn is about 673 K, which is much lower than that for Mg 2 Si, and the oxide layer grows linearly with time. 27,89Adding Sn to Mg 2 Si can improve the TE performance, but it also makes the materials more susceptible to oxidation due to the low melting point of tin at about 505 K.The oxidation rate of the Mg 2 Si 1−x Sn x alloys (x = 0.1−0.6)was slow for temperatures below 703 K, but breakaway oxidation occurred at higher temperature ranges, and the onset temperature decreased with increasing levels of Sn in the alloy. 27,90Mg 2 Si 1−x Sn x powders started decomposing into MgO, Si and Sn at 630 K; whereas the dense pellets decomposed at a significantly slower rate compared to the powder samples due to their smaller specific surface area. 90In the meantime, the oxidation resulted in the redistribution of Mg (Figure 4b), which could compromise the performance of the TE.Further investigation suggested that a nonprotective MgO layer and the Sn-rich liquid at the interface led to the breakaway oxide layer. 27Mg 2 Si 0.4 Sn 0.6 pellets oxidized easily and disintegrated into powders after heating at 823 K for 12 h in air. 91It also oxidized in an inert N 2 gas atmosphere.Although no obvious structural change in the slightly oxidized Mg 2 Si 0.4 Sn 0.6 sample at lower temperatures, the carrier concentration reduced clearly since oxidation created Mg vacancies in the lattice. 91We also found that the Mg 2 Si/Sn alloy pellet became dark, segregated, and distorted after being heated to 973 K for 50 h (Figure 4c).Mg 2 Ge also tends to absorb moisture from the atmosphere/humid air leading to its decomposition, which requires special care in storage. 92g 3 (Sb,Bi) 2 -based materials also exhibit weak stability, including Mg oxidization and loss, decomposition at high temperatures, as well as possible deliquescence in humid environments.The relative instability of the Mg 2+ lattice (in octahedral site) can be attributed to the small ionic radius.Increasing the Bi content in Mg 3 Sb 2−x Bi x would weaken interlayer bonding and result in poorer thermal stability.93 Additionally, at elevated temperatures, the precipitation of the Sb/Bi phase and Mg loss can be observed, influenced by the high vapor pressure of Mg.MgAgSb is well-known to exist in three phases between 300 and 693 K. α-MgAgSb possesses a tetragonal structure with a distorted rock-salt lattice.and remains stable up to 573 K, demonstrating decent TE properties.92 However, secondary phases and impurities like Ag 3 Sb and pure antimony in the system often compromise performance.Intermediate temperature β-MgAgSb is stable up to 633 K, above which it transforms to a high-temperature phase (γ-MgAgSb).49,52 Both of these phases are not favorable for the TE function.53 3.2.2.Sublimation and Dissociation.While heating a TE couple in an inert atmosphere can prevent oxidation, sublimation of species such as Sb from TE materials at elevated temperatures also causes performance degradation.Indeed, TE materials containing elements with high vapor pressure such as Pb, Ge, Te, Sb, Sn, etc., typically exhibit high sublimation rates at elevated temperatures.71 For instance, when heated to 773 K in a vacuum, the Mg 2 Si 0.3 Sn 0.7 solid solution experiences significant Mg loss due to the high vapor pressure of Mg.Conversely, when heated in air, the sample oxidizes.94 For the Mg 2 (Si−Sn) materials, above a certain ignition temperature, the passivating outer layer of MgO breaks down, and oxidation proceeds exponentially due to the formation of liquid Sn below the MgO layer.27 The TE materials also exhibited decreased carrier mobility and carrier concentration due to the Mg loss and Sn precipitation during the heat treatment.
Mg 3 Sb 2−x Bi x alloys possess decent TE properties in the temperature range between 300 and 773 K; however, their thermal stability remains a problem.Approximately 11 wt % elemental bismuth in the N-type Mg 3 (Bi,Sb) 2 crystallized as a secondary phase after the first heating cycle from 300 to 725 K, leading to the decomposition of the compound. 9 5Bismuth was released from the N-type  5a). 96,97At a temperature of 773 K for 6 h, the average concentration of Mg in the Mg 3 Sb 2−x Bi x alloys changed from 63.61 at% to 52.43 at %. indicating a significant loss of Mg. 97 At a vapor pressure of 10 Pa, the corresponding temperatures for Mg, Sb, and Bi are 773, 876, and 1041 K, respectively.The low melting temperature of Mg accounts for its loss at this temperature.Meanwhile, Sb loss was also observed. 97.2.3.Materials Miscibility.Mg 2 Si 1−x Sn x materials have decent TE performance within the intermediate temperature range (300−773 K), heavily influenced by the ratio of Si to Sn. Above 773 K, Mg 2 (Si,Sn) material system suffers from Mg loss.However, Mg 2 Si and Mg 2 Sn are not completely miscible, and an immiscibility region typically occurs for x values between 0.4 and 0.6.94 It is commonly reported that within the miscibility gap, phase separation occurs followed by the formation of elemental Si and Sn, or an Sn−Mg melt due to the loss of Mg from Mg 2 (Si,Sn).98 Due to the miscibility gap, separation into Sn-rich and Si-rich phases could occur in Mg 2 (Si,Sn) during heating and cooling, leading to expansion and porosity resulting from Kirkendall effect in the materials, as displayed in Figure 5b.30,99 These processes are detrimental to Mg 2 (Si,Sn) because they signify low thermal stability of the material system, primarily due to the loss of Mg.

Unreliable Contact and High Contact
Resistance.Direct soldering of most TE materials poses challenges due to either poor wettability of solder on TE materials or subsequent reaction/diffusion between the materials at elevated temperatures.Other issues include diffusion and self-diffusion in TE materials and contact structures, as well as the formation of intermetallic compounds in contact structures, resulting in unreliable contact between the TE leg and the electrode. 100The choice of different electrode materials further complicates the production of the TE module, including electrode fabrication, interface optimization, and protective coating.The reliability and compatibility of the metallized contact layer on both TE materials pose a significant challenge for constructing TE modules.
Degradation of contact structures primarily occurs due to interfacial reactions at the junction between the contact material and the semiconductor materials of the TE leg at high temperatures.For example, the electrical resistance between the Mg 2 Si TE leg and the Cu electrodes was around 0.7 Ω after production; however, after 1 h of treatment at 973 K, the contact resistance increased to almost 18 Ω which was a 25-fold increase, mainly originating from the oxidation of the TE leg. 86The formation of intermetallic compounds due to these reactions markedly raises the electrical resistivity of the contacts.These intermetallic compounds, being brittle, develop a porous structure over time when subjected to temperature gradients, which leads to cracking.This not only increases electrical contact resistance but also causes the breakdown of contact structures.In the case of soldered contacts, the solder contents can penetrate the TE material, compromising its performance.Another issue is the diffusion of elements in TE materials and the contact materials, which can lead to the formation of precipitates, as well as the creation of structural defects, especially dislocations.Diffuse degradation resulted in the deteriorating TE properties and a reduction of their mechanical strength.As shown in Figure 6, a TEG comprising two legs of high manganese silicide (HMS) and two legs of Mg 2 Si 0.55 Sn 0.45 was assembled by soldering the TE legs to metallized ceramic plates (AlN-DBC) using silver solder.A thin layer of Au/Ti (300/100 nm) served as a diffusion barrier between the (N & P)-type and the solder.After 100 h (400 cycles) of operating, fractures (cracks) were localized on the hot side of the Mg 2 Si 0.55 Sn 0.45 legs.During thermal testing, the internal electric resistance of the TEG increased, likely due to the formation of a thin layer of MgO and a diffusion layer between the TE leg and the solder (Ag) on the hot side of the legs. 99n additions to the properties of TE materials, the practical energy conversion efficiency and service life of TE devices are highly determined by the assembling process and the contact interface.Liu summarized the challenges and the interrelationship among different requirements and designs for the interface contact, as shown in Figure 6c. 101Therefore it is crucial to achieve a reliable contact through special pretreatment of the semiconductor surface with desirable roughness, the creation of an anti-diffusion/barrier layer, and the application of a sublayer/adhesion layer that enhances the bond strength of the contact structure with the TE material. 66

ENHANCEMENTS OF THE PERFORMANCE OF THE TE MATERIALS/MODULES
To address the degradation of TE materials and modules, various approaches have been attempted to protect them and enhance their performance and durability.In this section, we will address the main points regarding strengthening mechanical strength, protecting the TE module from degradation, and improving contact for better performance of TE modules.

Strengthening Mechanical Strength.
While the toughness of silicide-based TE materials is relatively higher than that of other TE materials, it remains as brittle as glass.Therefore, enhancing fracture toughness is essential to prevent or deflect propagating cracks induced by external loads and thermal stress.4.1.1.Grain Refinement.Grain refinement is an effective technique for enhancing the toughness and strength of brittle polycrystalline materials: the smaller the grain size, the stronger the mechanical properties become.Nanostructuring has been found to have a beneficial effect on the physicochemical and mechanical properties of the Mg 2 Si−Mg 2 Sn solid solutions, resulting in a substantial increase in mechanical strength and resistance to oxidation.However, further decreasing the grain size to the nanoscale does not influence the TE efficiency in the operating temperature range. 34onventional milling techniques such as planetary and vibratory ball milling have been utilized to reduce grain size.Schmidt et al. examined Mg 2 Si processed by powder metallurgy and sintered via pulsed electrical current sintering. 103As the mean grain size reduced from 3.9 to 2.4 μm, the Vickers hardness and fracture toughness increased from HV 1.0 5.0 GPa and K IC(9.8N) of 0.9 MPa•m 1/2 to HV 1.0 5.4 GPa and K IC(9.8N) of 1.3 MPa•m 1/2 .Wang prepared nanocrystalline Mg 2 Si intermetallics (d ≈ 54 nm) using mechanically activated solid-state reaction plus hot-pressing, and its toughness reached 1.67 MPa•m 1/2 , which is the highest value reported in the literature. 104The nanostructured MgAgSb TE materials with a grain size of about 150 nm, prepared by ball milling and hot press process, exhibited significantly strong mechanical properties.As shown in Table 2, Young's modulus, nanoindentation hardness, compressive strength, and fracture toughness are 55.0 GPa, 3.3 GPa, 389.6 MPa, and 1.1 MPa•m 1/2 respectively. 105heoretical analyses have suggested that grain refinement becomes effective when the grain size is reduced to less than 100 nm.However, it noted that as the grain diameter decreases, oxidation becomes more prevalent due to the significantly larger specific surfaces and grain boundaries in fine grains.Producing bulk materials with nanosized grains is challenging because surface oxidation can lead to impurity formation, particularly MgO. 112De Boor et al. found that even a small amount (7 wt %) of MgO can result in a significant reduction (30%) in ZT.Thus, they recommended optimizing processing parameters such as grain size, process temperature, and sintering temperature to fabricate pure Mg 2 Si without impurities.
4.1.2.Addition of Second Phases.The addition of second phases such as fibers, whiskers, flakes, and particles has proven effective in enhancing toughness.For particulate composites with a volume fraction of ∼10%, toughness can reach up to 2.2 MPa•m 1/2 . 73anophases with high fracture toughness, such as SiC, CNTs (carbon nanotubes), or graphene/graphene oxides, have been extensively investigated for their potential in improving the fracture toughness of TE materials. 78chmidt et al. discovered that incorporating ∼2 vol % SiC nanoparticles via a planetary ball mill enhanced the fracture toughness of Mg 2 Si by a third, while the Young's modulus (∼112 GPa) and hardness (∼4.8 GPa) remained relatively insensitive to the addition of 0−4 vol % SiC nanoparticles. 107This improvement in fracture toughness and the compressive strength was attributed to the pinning effect, fiber bridging, and fiber pull-out mechanisms in the Mg 2.16 (Si 0.3 Sn 0.7 ) 0.98 Sb 0.02 composite with 0.8 at% SiC nanopowders or nanowires, resulting in enhancements of about 50% and 30%, respectively (Table 2).A maximum ZT value of 1.2−1.3 was achieved at 750 K. 16 Moreover, the flexural strength and Vickers hardness of the composites at room temperature were also enhanced to various degrees.Inoue et al. introduced 10 vol % SiC into the Mg 2 Si grains using a plasma-activated sintering process, resulting in a toughness of 1.02 MPa•m 1/2 for the intragranular Mg 2 Si/SiC composite, which was 60% higher than that of pure Mg 2 Si, with only a small reduction in electrical conductivity. 106he incorporation of a small quantity (0.25−1 vol %) of conductive glass-frit leads to a notable enhancement in the mechanical properties of the mechanically alloyed and hot-pressed Mg 2 Si by eradicating microcracks inherent in the brittle Mg 2 Si system. 76Al-doped samples containing conductive glass-frit achieved a power factor times temperature ( As demonstrated in Figure 7, different concentrations of graphene oxide nanosheets (GOs) and multiwalled carbon nanotubes (MWCNs) were hot pressed together with Mg 2 (Si 0.3 Sn 0.7 ) 0.99 Sb 0.01 powders to fabricate TE composites.A significant improvement, with a 27% in flexural strength and a 41% in fracture toughness through crack bridging, was achieved without compromising the TE properties when 75%GOs/25%MWCNs were added. 110However, careful consideration is needed when incorporating nanoparticles as they can potentially reduce TE performance. 113By simultaneously activating three different inhibition mechanisms for crack propaga-tion�bridging of cracks, sheet pullout within the crack, and deflection of crack propagation�the incorporation of dual nanoinclusions of reduced graphene oxides (rGOs) and Sn NPs (50−150 nm) into Al and Bi codoped Mg 2 Si TE materials enhanced fracture toughness to 2.26 MPa•m 1/2 from 0.82 MPa•m 1/2 for pristine Mg 1.96 Al 0.04 Si 0.97 Bi 0.03 .However, the TE performance declined due to the deterioration of electronic transport properties stemming from enhanced electron scattering. 113he addition of ZrO 2 microparticles into Mg 3.2 Sb 1.99 Te 0.01 increased the compressive and bending strengths to 669 and 269 MPa, respectively, from 565 and 193 MPa. 111 Furthermore, the  combined secondary phase reduced lattice thermal conductivity and increased electrical conductivity, albeit with a slight degradation in the Seebeck coefficient.The average ZT in the temperature range of 300 to 500 K reached 0.8.4.1.3.Summary.The use of nontraditional processing technologies, such as pressure-induced sintering, can strengthen the fracture toughness; however, its effect is limited to the intrinsic property of the materials.Grain refinement is an effective method to reinforce the toughness, but the process must be optimized to avoid excess oxidation caused by the increased specific area.The addition of second phases attracts much interest due to its significant impact, but care must be taken to avoid adversely influencing the thermoelectric properties of the materials.

Protection of the TE Legs or Modules. 4.2.1. General Methods of Protection.
The major factors affecting the stability of TE materials are the atmosphere and elevated temperature.One solution is to seal the TEG devices with an inert gas like argon or to operate in a vacuum, which reduces oxidation.Kambe et al. encapsulated the SiGe or BiTe TE modules in a vacuum-tight stainless-steel container to prevent oxidation at intermediate temperatures (from 573 to 973 K) during power generation. 114Salvador et al. encapsulated the Skutterudite-based TE modules in aerogels via the Sol−Gel method by casting an ortho-organo-silicon-based sol mixture and catalyzing a condensation reaction to form a silicon oxide gel. 115However, the significantly rising costs of these metallic housing structures and the additional reliability issues of long-term sealing make this technology difficult to apply.Meanwhile, the sublimation of volatile alloying components and impurities occurs most extensively in a vacuum, increasing with the temperature.Safety issues arise from using inert gas to protect the TE modules, generating extra cost and making the system more complex, thus hindering its employment.
From a materials design point of view, oxidizing-resistant materials can be developed by modifying the fabrication route to improve the microstructure, thereby increasing the stability of the materials without the need for protective environments. 66For example, Mg 2 Si prepared by an all-molten method with no residual metallic-Mg exhibited atmospheric durability at 873 K for 1000 h. 73Others suggested that increasing the nominal content of Mg can improve thermal stability but it can lead to higher thermal conductivity and lower oxidation resistance. 116−119 Due to the harsh working conditions, protective coatings for high-temperature applications should incorporate chemical stability, wetting properties, and thermal expansion compatibility.They should have good adhesion to TE materials and serve as an effective diffusion barrier against gases and/or liquids.Additionally, they should maintain stability when in contact with chemical agents. 120he most common coatings for high-temperature TE applications include oxides, nitrides, silicides, borides, carbides, or their mixtures. 1217][118][119]122,123 Silica-based glass and glass− ceramic coatings have been used to protect PbTe up to 773 K, 124 MnSi up to 873 K 125 and Bi 2 Te 3 -based TE materials as well. 126 DC magntron sputtering (MS) AlTiN up to 2.6 μm can protect the Ni− Zn tetrahedrite well up to 723 K. 127 However, there are limited reports of the coatings applied to Mg-based TE materials or modules.Table 3 demonstrates some research on improving the oxidation resistance and the thermal stability of Mg-based TE materials.91 The dense, well-adherent and uniform Al 2 O 3 -coated Mg 2 Si 1−x Sn x pellets remained stable in inert gases at 823 K for 12 h, while the unprotected sample decomposed completely into MgO, Si and Sn under the same operating conditions.Surface passivation coating, aimed at blocking oxygen transmission, was one method used to deactivate the surface of Mg 2 Si.Initially, the Mg 2 Si surface was stabilized by forming a 10-μm thick surface passivation layer using an alkaline conditioner. Subquently, the surface was dip-coated with a coating agent consisting of SiO 2 , ZrO 2 , and mica. 73 fter aging at 873 K for 7000 h in air, the Mg 2 Si TE chips remained durable with stable resistivity at 3.45−3.73× 10 −6 Ω•m.
The Mg 2 Si samples were coated with 9 μm SiO 2 layer using a modified Sol−Gel route via dip-coating, followed by drying at room temperature and annealing in a vacuum for 1 h at 573 K.The silicacoated Mg 2 Si displayed excellent structural and TE properties, maintaining stability for up to 200 h at 823 K in air.In contrast, the surface of the uncoated pellets degraded, forming cracks after 30 h and crumbled after 200 h of aging. 129urthermore, Park et al. tested different oxide coatings such as plasma sprayed alumina (Al 2 O 3 ), yttria (Y 2 O 3 ), 8 mol % yttriastabilized zirconia ((Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 , YSZ), and plasma-nano coating deposited 200 nm of the 20 mol % samaria-doped ceria (Sm 0.2 Ce 0.8 O 1.9 , SDC).They found that the different CTE led to the cracks in the 50 μm thick alumina and yttria coating after heating to 973 K for 1 h.After this treatment, some local delamination occurred in the thin SDC coating, while the YSZ remained intact.YSZ exhibited excellent oxidation suppression characteristics for Mg 2 Si after 10 thermal cycles from room temperature to 873 K (Figure 8a,b). 86Coated Mg 2 Si TE leg with YSZ was also effective in stabilizing the contact and the electrode, as the contact resistance remained low at 0.7 Ω after heat treatment. 86 glass coating, containing a mixture of oxides including SiO 2 , K 2 O, Na 2 O, CaO, MgO, Al 2 O 3 , B 2 O 3 , was applied onto pellets of Mg 2 Si 0.487 Sn 0.5 Sb 0.013 , and fired at 823 K for 1 h.130 The glass coating exhibited a CTE of approximately 17 × 10 −6 K −1 , slightly lower than that of the substrate (17.6 × 10 −6 K −1 ), resulting in a moderate compression state in the coating.This characteristic potentially enhanced the resistance of the coated Sb-doped Mg 2 (Si,Sn) to crack propagation during cooling process or thermal cycling.Following an aging test at 773 K for 120 h in air, the uncoated Mg 2 Si 0.487 Sn 0.5 Sb 0.013 sample underwent complete oxidation, transforming into a mixture of powders including MgO, SnO 2 , SnO, Sn and Si, whereas the glass coated sample appeared unaffected.Nevertheless, further investigations regarding interface alterations and TE properties are necessary before application.120 Silicon-oxycarbide (SiOC), often referred to black glass, denotes a carbon-containing silicate glass in which oxygen and carbon atoms form bonds with silicon within an amorphous network structure.This materials boasts excellent mechanical properties and chemical stability.135 Magnesium silicide samples were covered with black glass (SiOC) amorphous coatings using dip-coating (sol−gel) method, followed by annealing in Ar gas atmospheres at various temperatures between 673 and 823 K.The protective coating exhibited continuity and good adhesion up to 723 K.However, at higher temperatures, it began to crack due to differences in CTEs between the coating and TE material.131 4.2.3.Silicide and Similar Compounds. MoS 2 has a high melting point (2303 K), along with excellent oxidation resistance and diffusion barrier characteristics.Consequently, it has found application in high-temperature structural materials such as combustion chamber components, heating elements in oxidizing environments, and diffusion barriers in microelectronic devices. 136A 2.5 μm thick MoSi 2 thin film barrier, deposited via RF magnetron sputtering, demonstrated good thermo-mechanical compatibility with the sintered Mg 2 Si pellet substrate.It efficiently protected the substrate up to 873 K. To maintain the Seebeck potential of the pellets, an insulating nanocomposite MoO 3 /SiO 2 film was utilized between the substrate and the MoSi 2 protective layer, aiming to reduce interference from the conductive coating.However, considerations regarding CTE mismatch and compositional instability arose at temperatures exceeding 773 K. 132 β-FeSi 2 , an environment-friendly silicide semiconductor with an orthorhombic structure, exhibited remarkable oxidation resistance in the high-temperature region below 1073 K. 137 RF magnetron sputtered β-FeSi 2 on Mg 2 Si improved oxidation resistance up to 873 K. 87 Upon heat treatment in air at 873 K for 3 h, an 8-μm thick oxide layer formed on uncoated Mg 2 Si samples.Conversely, Mg 2 Si samples coated with 0.7 μm thick β-FeSi 2 films showed no oxide layer formation.
Recently, we investigated the thermal stability of the compact and uniform CrSi x coatings deposited using a close field unbalanced magnetron sputtering technique applied to hot-pressed sintered Mg 2 Si 0.888 Sn 0.1 Sb 0.012 TE pellets.The uncoated pellet oxidized and developed cracks on the surface region after 10 cycles of heat treatment at 773 K for 1 h (Figure 8c).In contrast, the CrSi-coated pellet remained intact and exhibited integrity after 50 cycles of test, indicating excellent stability (Figure 8d).

Other Compounds.
Boron nitride (BN) coating, applied to Mg 2 Si 0.3 Sn 0.7 pellets by spraying a 0.5 mm layer of BN, provided effective protection up to 773 K.However, at 823 K, the carrier density in the sample deteriorated. 94Mg 3 Sb 2−x Bi x alloys tend to become unstable at temperature above 673 K, experiencing significant Mg loss and altered microstructures.Coating Mg 3 Sb 2−x Bi x alloys with BN effectively suppressed Mg loss, greatly enhancing their thermal stability. 97A commercial Mg−Mn alloy (ME20M) was used as protective coating layer for Mg 3 Sb 1.5 Bi 0.5 TE leg.The dense and continuous ME20M-coated Mg 3 Sb 1.5 Bi 0.5 sample remained stable for nearly 30 days at 673 K, which could effectively obstruct Mg escape and prevent oxygen penetration. 134 solvent-based resin (CP4040-S1, ARAMCO Scientific Company, Los Angeles, U.S.A.) was brushed onto Mg-based TE pellets and cured for 45 min at 523 K.After aging in air at 773 K for 120 h, the uncoated Mg 2.1 Si 0.487 Sn 0.5 Sb 0.13 completely burned, turning into a powder consisting of various compounds (MgO, SiO, SnO 2 , and Sn).In contrast, the resin coated pellet, with a thickness of 30−100 μm, did not experience significant oxidation and remained mainly composed of a single phase, although some cracks and small amount of MgO were present due to imperfections in the coating.133 4.2.5.Coating Selection Principles and Summary.In short, the selections of coating should adhere to the following principles: 120 Low thermal conductivity to minimize parasitic heat loss.Low electrical conductivity to prevent short-circuiting.Coefficient(s) of thermal expansion matching that of the TE material, ensuring good interface compatibility and stress resistance during thermal cycling.Sufficient thermal stability and mechanical strength for longterm durability.No adverse impact on TE performance.
Oxides like silica/glass, silicides, and nitrides are the most used, and production methods include dip coating, Sol−Gel, plasma spraying, liquid spraying, atomic-layer deposition (ALD), and PVD etc. Siliconbased oxides and silicide are preferred coatings for the Mg 2 Si-based TE modules.While magnetron sputtering offers high-quality coatings, it comes with a higher cost.Plasma spraying, dip coating, and Sol−Gel techniques are more economical alternatives.
4.3.Improving the Contacts.TE module degrades over longterm, especially on the hot side, where interdiffusion and material loss are more prevalent.Metallization of the TE leg or collecting plate is of great importance by introducing functional layers, including a diffusion barrier layer, a contact layer, an adhesion layer and a compliant layer.The metallized layer assists the soldering process or reduces mutual diffusion.Due to the harsh working conditions, the ideal contact material should exhibit the following characteristics: 1,138 high electrical conductivity and good thermal conductivity, CTE matching with the TE elements, capability to be made very thin to minimize total electrical and thermal resistances, low contact resistance at the interface between the contact layer and the TE surface, stability at elevated operating temperature, ability to form strong mechanical bonds with the TE layer, higher yield strength than solder at working temperature, and the electrode should have a melting point 20% to 50% higher than the joining temperature. 139he linear coefficient of thermal expansion (CTE) for Mg 2 Si can be expressed in the form: 140 Mg 2 (Si,Sn) based materials have a slightly higher CTE of 16.5−18.5× 10 −6 K −1 . 130,141s shown in Table 4, metals like Cu, Ni, Ag, Al, Mo, Ti, Au, Pd, and their alloys are potential candidates for bonding with Mg-based TE materials, due to their low-resistivity and higher bond strength (4.3−17.3MPa).Lower interfacial resistance of multielement alloys, such as (Co, Cr, Ti)Si 2 , Ni 45 Cu 55 , 304 stainless steel (304SS), and Mg 2 SiNi 3 , have also been reported. 142. 3  148 The adhesion of the sputtering deposited Cu film electrodes with a postannealing treatment showed the best adhesion performance and lower contact resistance than that of the Ni and Au layers.A 300-μm Cu sheet brazed on the N-type Mg 2 Si pellet by Ag 56 Cu 22 Zn 17 Sn 5 alloy demonstrated the lowest contact resistances of 4.43 × 10 −5 Ω•cm 2 compared to the sputter-deposited contact layer. 148Cu electrodes were bonded to Mg 2 Si using the spark plasma sintering (SPS), which formed a 10-μm thick intermediate layer with Mg, Si, and Cu diffused into each other.The Mg 2 Si/Cu joints remained intact after annealing at 773 K for 72 h in vacuum, but the contact resistance increased with cracks developing along the boundary after aging at higher temperatures (823 K and 853 K). 149 Cu foil can be joined with N-and P-type Mg 2 Si 0.3 Sn 0.7 legs through direct or indirect resistive heating.Cu diffused into TE materials, creating relatively thick (200 and 100 μm, respectively) and complex reaction layers under both conditions.Electrical contact resistance remained less than 1 × 10 −5 Ω•cm 2 even after annealing.100 Ayachi et al. joined the Cu and Ni 45 Cu 55 contacting electrodes to Mg 2 Si 0.3 Sn 0.7 pellets by hot pressing in a current-assisted press.150 They found that Ni 45 Cu 55 joining showed relatively low contact resistance of ∼3 × 10 −5 Ω•cm 2 but had a less inhomogeneous reaction layer, while Cu joins had much lower specific electrical contact resistance for both Nand P-type silicides (<1 × 10 −5 Ω•cm 2 ) with a wide, highly conductive diffusion regions.After annealing at 723 K for 1 week, the resistance values of the Cu joint increased up to ∼1 × 10 −4 Ω•cm 2 for annealed N-type samples but remained low (<1 × 10 −5 Ω•cm 2 ) for P-type.150 Cu can diffuse into the TE materials and react with them, causing a negative effect on the TE performance and occasionally leading to local delamination of the electrode.151 To reduce Cu diffusion during sintering, SS 304 interlayer was used between Mg 2 Si 0.4 Sn 0.6 /Cu contacts.152 Without the SS 304 layer, the electrical resistivity of Mg 2 Si 0.4 Sn 0.6 increased by ∼60% due to Cu diffusion during sintering.The Cu/SS 304/Mg 2 Si 0.4 Sn 0.6 contact fabricated by one-step hot press sintering had a specific contact resistance of ∼6.1 × 10 −6 Ω•cm 2 and remained <1 × 10 −5 Ω•cm 2 even after 15 days annealing at 723 K. Al and Ti interlayers were also used between the sputtering deposited Cu film and the Mg 2 Si semiconductor, but they weakened the electrode adhesion and increased the contact resistance.148 To reinforce the electrical conductivity of the Nb/Ta doped Mg 3 (Sb,Bi) 2 TE leg, Fe, Mg turnings, and Cr powder were mixed and ball milled as TE interface powders materials.44 They were then sandwiched with Cu powders in a graphite die and sintered at 873 K for 10 min under a pressure of 50 MPa to produce a Cu/FeMgCr interfacial contact of 1.5 mm thick.The reduced interfacial barriers are conductive to carrier transport at low and high temperatures.
4.3.2.Nickel Contact.Nickel has a relatively low contact resistance in the order of 10 −5 Ω•cm 2 and a closed CTE. 153It is commonly used as the electrode material for Mg 2 Si as it is durable and does not react significantly with Mg 2 Si at the working temperature. 71Direct hotpress bonding methods were used for leg preparation using Mg, Si,  and Ni powders to form Mg 2 Si and nickel contact electrodes.The lowest contact resistances of Mg 2 Si samples with Ni electrodes was about 1.0 × 10 −5 Ω•cm 2 .An intermediate layer consisting of different ternary phases of Mg/Si/Ni and Si 12 Ni 31 was formed, which had good adhesion to both the Ni electrode and the Mg 2 Si. 154However, complex new phases formed at the Ni/Mg 2 Si interface complicated the relationship between processing and the contact resistance of the product. 155Although the joints had a decent shear strength of about 20−26 MPa, the contact resistance was also high at 1.28−1.44× 10 −3 Ω•cm 2 . 155After long-term annealing at 723 K for 600 h, the shear strength was 23 MPa while the contact resistance increased further to 3.7 × 10 −3 Ω•cm 2 .These values are higher than the reported specific contact resistivity between Ni and commercial Mg 2 Si (∼10 −4 Ω• cm 2 ). 148,156he method of joining the presintered TE pellets with electrodes has an advantage over the co-sintering of TE and contact powder material together to fabricate the contact, as it allows for a disentanglement of the sintering temperature and the joining temperature.Interface thickness and composition can be tuned to reduce the contact resistance.A specific contact resistance (2.5−5 × 10 −5 Ω•cm 2 ) was obtained for the Ni electrode joined with both Nand P-type Mg 2 Si 1−x Sn x at a temperature lower than 973 K. 141 Another method to obtain a better contact is achieved by pressing a thin nickel foil with powders instead of direct sintering of nickel powders on top of Mg 2 Si.The contact utilizing a nickel foil with Bidoped Mg 2 Si TE powders was fabricated via an Induction Assisted Rapid Monoblock Sintering Technique, and the contact resistance was reduced to about 1.4 × 10 −5 Ω•cm 2 . 157By pressing a thin nickel foil onto the surface of the leg during sintering, the contact resistance for Ni/Mg 2 Si 0.98 Bi 0.02 junctions was only 5 × 10 −6 Ω•cm 2 . 158evertheless, care must be taken, especially for Ni electrodes, as cracks are easily formed due to the brittle Mg 2 Si 1−x Sn x not accommodating well with Ni, given the slightly larger difference in CTE (17.5 vs 13 × 10 −6 K −1 ). 141A mixture of the TE material (like Mg 2 Si 0.3 Sn 0.7 ) and the contact metal powders (Cu/Ni) can be used as an intermediate diffusion barrier between the metal foil contact and TE materials. 159The electrical contact resistances of the joints compacted using the monoblock sintering technique were 3 and 19 × 10 −5 Ω•cm 2 for N-and P-type legs, respectively, and they remained stable after annealing at 673 K for 7 days.Mg 2 SiNi 3 was also used as a diffusion barrier material between Ni and Mg 2 Si-based TE material. 160owever, the migration of the Mg atom between Mg 2 Si to Mg 2 SiNi 3 led to performance deterioration.Tohei et al. utilized aluminum instead of a silver-alloy braze to bond Mg 2 Si to Ni and reported a shear strength of 19 MPa; however, its thermal stability and contact resistance were not clear. 161Chromium layer was also used as a diffusion-impervious layer slowing down Ni diffusion and improving the mechanical properties of the contact. 162ickel layers were electroplated onto the Mg 3 (Sb 1−x Bi x ) 2 TE legs to facilitate the soldering and acting as an interfacial layer.However, the conductivity reduced after long service time. 163Nickel contact hot-pressed on the as-prepared Mg 3+δ Bi 1.5 Sb 0.5 had a resistance of 1.30 × 10 −5 Ω•cm 2 , which increased to 1.85 × 10 −5 Ω•cm 2 after aging for 2100 h at 573 K. 163,164 Mixtures of Ni with other metals like Fe or Cr also showed promising results, especially NiFe, which exhibited excellent thermal stability and the lowest ohmic contact resistance even after aging (1.30 × 10 −5 Ω•cm 2 ) due to the formation of metallic NiMgBi between NiFe and Mg 3+δ Bi 1.5 Sb 0.5 .

Other Metals and Compounds.
Silver nanoparticles soldered at low temperature (573 K) can sustain high service temperatures (>1000K), demonstrating stable performance and no degradation for various TE generators (such as Bi 2 Te 3 -based, PbTebased, and half-Heusler-based) operating across a wide range of temperatures. 165Ag electrode joining at a temperature of 723 K with N-and P-type Mg 2 Si 1−x Sn x had a low specific contact resistance of 0.9−1.5 × 10 −5 Ω•cm 2 . 141However, joining silver at higher temperatures like 873 K led to degraded samples due to the formation of a liquid phase.The formation of Ag defects in the solid solution lattice during the diffusion process also causes some concerns. 166Ag and α-MgAgSb have almost identical CTE (Table 4), which can thus reduce the interface stress significantly. 167Ag has excellent electrical and thermal conductivity and is easy to solder due to its softness; the small concentration gradient could decrease the elemental diffusion.A single α-MgAgSb leg with Ag pads on both the bottom and top surfaces was produced using a one-step hot-press technique, resulting in high mechanical strength and low resistance below 10 −5 Ω•cm 2 . 168s an electrode or barrier material, silver is expensive; however, it has a low wetting contact angle, which helps the surface stick to the bonding alloys.A N-type segmented leg of Bi-doped Mg 2 Si (hot-side) bonded with Bi 2 Te 3 (cold side) was fabricated via evaporation deposited 1 nm Ag and 50 nm Ti. 169 In this design, silver (Ag) served as a "glue", while titanium (Ti) enhanced adhesion between the semiconductors.The inclusion of Ti and Ag adhesive layers ensured lower contact resistance and strong bonding.
Aluminum is a poor dopant, with a larger CTE than the TE material.However, it is malleable, which helps to accommodate the mechanical stresses due to the CTE difference.Camut found that aluminum bonded well to P-type and N-type Mg 2 (Si,Sn), giving low electrical contact resistances (10 × 10 −6 Ω•cm 2 ), which were preserved or even lowered after annealing.The interface was clean and free of detrimental secondary phases. 151The low melting point of 933 K allows for direct bonding to the TE material.Al was used as a solder to join Mg 2 Si-based TE legs to Cu 148 or Ni 161 electrodes.The contact was established and mechanically strong, and no secondary phase was formed at the interface.
Skomedal et al. reported the bonding of a Mo electrode on the hot side of high-performance Mg 2 (Si 0 .4 Sn 0 .6 ) 0 .9 9 Sb 0 .0 1 and Mg 2 Si 0.53 Sn 0.4 Ge 0.05 Bi 0.01 , with thin layers of Pb, Ni, and Cr adopted to improve adhesion and contact. 153The best design exhibited an inner resistance in the region of 0.1 Ω at temperatures above 673 K.
Silicides such as TiSi 2 , CrSi 2 , CoSi, and NiSi exhibit low contact resistances with HMS in the order of 10 −6 −10 −5 Ω•cm 2 .They align with the empirical "like-bonds-like" rule with Mg 2 Si-based TE.They were simultaneously sintered together with Mg 2 Si to form an electrode with Ni as a binder layer, and their contact resistance decreased by about a third compared to the standard Ni electrode. 170 maximum output power of 153 mW was reached with CoSi 2 as the electrode, representing a 27% increase relative to the use of a nickel electrode at 600 K.
Other materials like Zirconium foil (16−125 μm) was used to bond skutterudite materials, acting as a diffusion barrier for Sb diffusion. 171However, whether this material can be used as a diffusion barrier on Sb containing Mg-based TE materials is still open for study.

Fabrication and Selection of Metal Contact.
There are various approaches to produce a contact layer on TE legs.For a thin layer (<10 μm), sputtering, electroplating, or chemical vapor deposition methods are employed to produce the contact material on a properly prepared TE leg.Each method offers different benefits in terms of precision, scalability, and cost.For a thick layer, hot pressing is applied to join the contact layer/foil to the TE material, which is relatively low cost and works effectively to create a diffusion barrier at elevated temperatures.If the two previously mentioned methods are not suitable, then conducting paste can be utilized to temporarily bond TE legs to the conducting strip.
In the testing of various metallic or silicide contacts with different Mg-based TE materials, Cu and Ni are commonly utilized as electrode or metalized materials.It has been observed that sintering Cu or Ni powder/plate separately with the TE pellets results in lower contact resistance.Additionally, interface engineering techniques such as transition or diffusion barrier layers may be employed to reduce contact resistance, prevent diffusion, and enhance the strength and durability of the electrode.Despite these advancements, the combined resistance from wiring and contact still contributes significantly, accounting for around 20−40% of the total module resistance.This poses a significant challenge and compromises the overall performance of the TE module.Consequently, achieving specific contact resistance below 1 × 10 −6 Ω•cm 2 remains a substantial challenge in optimizing contact layers for these materials. 71

PROGRESS OF TEG MODULES
Thanks to their low cost, abundant resources, and everincreasing performance, Mg-based TE devices are attracting growing interest and undergoing systematic investigations to overcome challenges for sustainable development toward higher power efficiency.In addition to optimizing the TE legs materials for the optimal performance, it is crucial to minimize degradation of TE device, especially over long periods, particularly on the hot side where interdiffusion and material loss are more prevalent.As illustrated in Figure 9, module design must prioritize approaches to improve stability of TE legs by either providing an inert environment or coating them.It is also important to reduce contact resistance and enhance joints by introducing functional layers between the TE leg and the current-collecting plate through metallization.These layers may include a diffusion barrier layer, a contact layer, an adhesion layer, and a compliant layer.
Currently, most of the research is concentrated on optimizing the TE properties; there is limited reporting on the TEG.TEGs require both N-and P-type materials to work efficiently, ideally with similar thermochemical and thermo-mechanical properties.In comparison with the well-developed N-type Mg 2 X(Si/Ge/Sn) materials with max ZT between 1.0 and 1.5, P-type Mg 2 X has much inferior performance with a max ZT just about 0.6 (Table 1), although it has a theoretical higher calculated max ZT value of 0.8. 18This mismatch requires TE modules to use Mg-based TE materials on a single leg or couple them with different TE materials such as high manganese silicide (HMS, max ZT of 1.0 176 ) or MgAgSb 177 (Table 1) to build TE generators.
Camut et al. produced a Mg 2 (Si,Sn)-based TEG with a power density of 0.9 W•cm −2 and 4% conversion efficiency at about 648 K. 174 A single-leg Mg 2 Sn 0.75 Ge 0.25 device using Cu 2 MgFe as contact layer exhibited a high power density of 2.6 W•cm −2 and conversion efficiency of 8% under a temperature difference of 643 K, which set a record-breaking value compared to other Mg 2 (Si, Ge, Sn)-based TE devices. 142A 8 mm thick N-type Nb/Ta doped Mg 3 (Sb,Bi) 2 single leg bond with Cu contact by FeMgCr interface layer demonstrated a room-temperature power factor >30 μW•cm −1 K −2 .Thanks to the reduced lattice thermal conductivity, a record-high average ZT > 1.5 was achieved, resulting in a high TE conversion efficiency of 15%. 44An α-MgAgSb single leg fabricated by a one-step hot-press technique, with Ag pads on both the bottom and top surfaces, exhibited the highest TE conversion efficiency of 8.5% reported so far, operating between 293 and 518 K. 168 At a current of 1.48 A, the power output reached 46.2 mW. 23 synergistic effort was undertaken to enhance the performance of Mg 3 Sb 1.5 Bi 0.5 TE module by utilizing FeCrTiMnMg alloys as interface contact layer and a MgMnbased alloy as the protective coating. 134The output power density was 1.7 W•cm −2 and a conversion efficiency of 13% for the single-leg device was achieved at a temperature difference of 495 K between 278 K and 773 K.In combination with the p-type commercial Bi 2 Te 3 , a two-couple TE device can generate power of 0.8 W•cm −2 with an energy conversion efficiency of 6%.
A π-shaped TE module, comprising Mg 2 Si and high manganese silicide (HMS), were brazed to a Cu electrode using an Ag−Cu−Sn−Zn alloy, while the remaining segments of TE materials were soldered together using a Pb−Ag−Sn−In alloy.A high-power output of 42.9 W/kg was achieved under a 771 K temperature difference. 169Skomedal et al. developed a TEG device composed of N-type Mg 2 Si 0.4 Sn 0.6 alloys and Ptype material MnSi 1.75 Ge 0.01 .They utilized Pb, Ni, and Cr layers to increase the bond strength and reduce the contact resistance of Mo electrode on the hot side. 153The optimal module exhibited an average power output of 0.37 W and a maximum power output of 3.24 W at 1008 K, with an estimated efficiency as high as 5.3%.Additionally, with N-type Mg 2 Si 0.4 Sn 0.6 and P-type MnSi 1.73 both doped with Sb, the constructed TE module demonstrated an efficiency of more than 6.5% under a temperature difference of 520 K. 23 In combination with N-type Mg 3 (Sb,Bi) 2 and P-type α-MgAgSb TE legs, the conversion efficiency above 7% is achievable at medium temperature range such as 548 K 178 and a record-high conversion efficiency of ∼7.3% is obtained at 593 K. 167 The conversion efficiency of the latest developed Mg-based TE single legs or paired P/N modules is summarized in Table 5, compared with those commercial Bi 2 Te 3 , PbTe devices.

FUTURE PERSPECTIVES
Although there has been significant progress in Mg-based TE materials and modules, ongoing efforts are still needed to further improve their performance and reliability.Some major areas for developments include: 1. Developing P-and N-type Mg-based compounds with matched ZT values by employing suitable alloying strategies and breakthrough fabrication methods/conditions to effectively manage the Fermi level and devise the band and phonon structures.2. While various techniques are applied to enhance toughness without reducing transport properties, such as grain refinement and the addition of a second phase, the lack of experimental data on long-term durability, creep, and fatigue for practical application remains a challenge.Collecting a large amount of data for statistical analysis is necessary.3. The long-term chemical stability of Mg-based TE materials and the power output stability of TE devices still face severe obstacles to conquer.Systematic studies are needed to ensure the future applicability of Mgbased TE materials.4. The strength of joints between TE materials and electrodes affects the mechanical properties of TEGs.
Optimization is needed to avoid thermal stress concentration and reduce the thickness of the diffusion layer.The geometry of TE legs and the method of arrangement in the module should be designed to restrict the movement of TE legs, for example, by using special frames or damping devices. 5. Advance in TE device fabrication is slower compared to the burgeoning fundamental research on TE materials, mainly due to the technical challenges associated with device design and assembly, particularly concerning electrode contact and interfacial design.Future efforts should involve holistic studies encompassing module design, material development, joint connection, fabrication method optimization, and sealing/protection of the Mg-based TE devices.6.A life cycle strategy from module design, materials choice, TE leg production, TEG assembly, and recycling should be proposed to conserve materials and resources for the benefit of the environment.

■ ASSOCIATED CONTENT Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.

Notes
The authors declare no competing financial interest.

Figure 2 .
Figure 2. A summary of the development of (a) N-type and (b) Ptype Mg-based TE materials with different alloying elements up to 2018. 17Reproduced with permission from ref 17, Copyright [2018], [RSC publishing].

Figure 4 .
Figure 4. (a) TGA analysis of SPS sintered Mg 2 Si pellet, with inset pictures of Mg 2 Si pellets before and after heat treatment at 973 K for 1h. 86Adapted with permission from ref 86, Copyright [2016] [Elsevier].(b) EDS line-scan of the 1 μm thick surface oxide layer on Mg 2 Si 0.4 Sn 0.6 Sb 0.01 after 48 h in air at 673 K.A noticeable depletion of Mg directly beneath the oxide layer is clearly identified; 27 Reproduced with permission from ref 27, Copyright [2016] [Elsevier].(c) Mg 2 Si 0.888 Sn 0.1 Sb 0.012 compound before and after treatment at 973 K/50h in our lab.

Figure 8 .
Figure 8.(a) SEM image of YSZ coated Mg 2 Si TE leg surface and (b) EDS oxygen distribution mapping after 10 thermal cycles between room temperature and 873 K; 86 adapted with permission from ref 86, Copyright [2016] [Elsevier].Thermal stability test for N-type Mg 2 Si 0.888 Sn 0.1 Sb 0.012 heating to 773 K for 1 h: (c) 10 cycles for as received Mg 2 Si 0.888 Sn 0.1 Sb 0.012 pellet, and (d) 50 cycles for PVD CrSi coated Mg 2 Si 0.888 Sn 0.1 Sb 0.012 pellet (our research).

Figure 9 .
Figure 9. Configuration of a practical TE module and the joining stacking layers between the TE leg and the ceramic plate.

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ABBREVIATIONS USED: ALD: Atomic Layer Deposition CTE: Coefficient of Thermal Expansion CVD: Chemical Vapor Deposition DC: Direct Current EDS: Energy-Dispersive X-ray Spectrometry HMS: High Manganese Silicide MS: Magnetron Sputtering PVD: Physical Vapor Deposition RF: Radio Frequency SDC: Samaria-doped Ceria

Table 1
summarizes the highest ZT values obtained for

Table 1 .
Reported Highest ZT of Mg-Based Thermoelectric Materials

Table 2 .
Mechanical Properties of Selected Mg-Based TE Materials

Table 3 .
Typical TE Material and Related Protection Coatings 4.2.2.Oxide Coatings.Al 2 O 3 thin film is a popular passivation material for silicon in the photovoltaic and microelectronic industries.Zhang et al. deposited a nanoscale amorphous Al 2 O 3 coating by ALD.
.1.Copper Contact.Copper has a CTE very close to that of Mg 2 Si-based TE material.Ferrario et al. compared the metals (Cu, Ni, and Au) contacts deposited by DC magnetron sputtering onto Mg 2 Si samples.

Table 4 .
Comparison of the Mg-Based TE Materials and the Metallic Electrodes32,73,143−147

Table 5 .
Reported Conversion Efficiency of Mg-Based TE Single Leg or Paired Module (Comprising P/N Materials) Compared with Other Modules